ENERGY IN GENERAL. 87 



instance of this in the reactions which take place 

 without the aid of external energy; and again, in 

 those very numerous cases which, such as the com- 

 bustion of hydrogen and carbon, or the decomposition 

 of explosives, the reactions continue when once primed. 

 I may make a further observation apropos of thermal 

 and photic energy. These are not two really and 

 essentially distinct forms, as was thought in the early 

 days of physics. When we consider things objectively, 

 there is absolutely no light without heat; light and 

 heat are one and the same agent According as it is 

 at this or that degree of its scale of magnitude, it 

 makes a stronger impression on the skin ^sensation of 

 heat) or on the retina (sensation of light} of man and 

 animals. The difference may be put down to the 

 diversity of the work and not to that of the agent. 

 The kinetic theory shows us that the agent is 

 qualitatively identical. The words heat and light 

 only express the chance of the meeting of the radiant 

 agent with a skin and a retina. At the lowest degree 

 of activity this agent exerts no action on the termina- 

 tions of the thermal cutaneous nerves, nor on the 

 optic nerve-terminations. As this degree is raised 

 the former of these nerves are affected 'cold, heat; and 

 are so to the exclusion of the nerves of vision. Then 

 they are both affected (sensation of heat and light), 

 and finally, beyond that, sight alone is affected. The 

 transformation of one energy into the other is there- 

 fore here reduced to the possibility of increasing or 

 decreasing the intensity of the action of this common 

 agent in the exact proportions suitable for passing 

 from one of the conditions to the other; and this is 

 easy when it is a question of going up the scale in the 

 case of light, and, on the contrary, it is not realizable 



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